1. Technical Field
The present invention relates to a lubrication apparatus for a four-stroke engine mounted in a portable working machine such as a brush cutter, a hedge trimmer and so forth. In the general meaning, a portable engine for a backpack working machine may be a kind of four-stroke engines.
2. Related Art
A lubrication apparatus for a four-stroke engine disclosed, for example, in Japanese Patent Application No. 2007-224824 has been known. In a lubrication apparatus for general four-stroke engines, a lubricating passageway is formed mainly in a cylinder block in order to reduce a four-stroke engine in size and weight.
When the lubricating passageway is formed in a cylinder near a combustion chamber, the oil flowing through the lubricating passageway may be heated more than necessary. If the oil heated excessively is supplied to a crank chamber, lubrication failure is likely to occur. In addition, the thickness of a cylinder block is increased by forming the lubricating passageway in the cylinder block, and rather this prevents the four-stroke engine from reducing its weight. Moreover, it is difficult to fabricate a long hole precisely in the thick portion of a cylinder block using a drill and so forth. This causes a drop in fabrication yield.